Device and System Having Water Microelectrolyzer Cells and Method of Using the Same

ABSTRACT

A device having a water vapor microelectrolyzer cell including microstructured parallel electrodes where the water splitting reactions can take place, the microstructured electrodes being covered by a thin layer of solid-state ion conducting material to allow for the conduction of protons during the device operation, while permitting the diffusion of water from the vapor phase into the electrodes, as well as the diffusion of evolved oxygen and hydrogen gases from the electrodes into the vapor phase environment above the ion-conductor film.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims foreign priority to International Patent Application PCT/IB2015/052338 filed on Mar. 30, 2015, the entire contents thereof herewith being incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to water electrolysis systems, devices, and methods, in particular, systems, devices, and methods that involve microfluidic approaches to hydrogen generation from water.

DISCUSSION OF THE BACKGROUND ART

The increase in the share of renewable energy sources for power production is essential for the significant decrease in the current levels of CO₂ emissions. See Chu et al., “Opportunities and challenges for a sustainable energy future,” Nature, 2012, Vol. 488, pp. 294-303, see also N. S. Lewis et al., “Powering the planet: Chemical challenges in solar energy utilization,” Proceedings of the National Academy of Sciences, 2006, Vol. 103, Iss. 43, pp. 15729-15735. Large amounts of investment have been provided to increase the capacities of solar and wind-based power generation. But power derived from these intermittent sources has brought significant challenges related to their incorporation into our current electricity distribution infrastructure, where electricity generation generally needs to match the demand. Water electrolyzers in particular can easily accommodate excess electricity by converting it into hydrogen fuel, which can be stored and subsequently used to regenerate electricity in a fuel cell, i.e. a reversible operating electrolyzer.

Classical electrolysis systems operate under alkaline electrolytes which allow the use of inexpensive and earth-abundant catalysts that operate under basic conditions. These alkaline systems pose significant corrosion challenges for all system components as they require the operation under strongly basic electrolytes. More recent approaches to water electrolysis follow the proton exchange membrane (PEM) approach using membrane-electrode assemblies (MEAs) based on ion-conducting polymers to perform the water splitting process. See Ayers et al., “Research Advances towards Low Cost, High Efficiency PEM Electrolysis,” Electrochemical Society (ECS) Transactions 2010, Vol. 33, Iss. 1, pp. 3-15. See also Carmo et al., “A comprehensive review on PEM water electrolysis,” International Journal of Hydrogen Energy 2013; Vol. 38, Iss. 12, pp. 4901-4934. See also Mark K. Debe, “Nanostructured Thin Film Electrocatalysts for PEM Fuel Cells—A Tutorial on the Fundamental Characteristics and Practical Properties of NSTF Catalysts,” Electrochemical Society (ECS) Transactions 2012, Vol. 45, pp. 47-68.

These systems have significant advantages because the ohmic losses through the polymer are minimized by the implementation of thin electrolyte layers, and they are fed with deionized water, alleviating most of the corrosion issues posed by alkaline electrolyzers. One alternative to using deionized water as the feed is to extract water directly from the vapor phase, which would simplify the operation of electrolyzers. Performing water electrolysis from the vapor phase exhibits several advantages: (i) lower water splitting potential, (ii) no additional transport overpotentials due to the formation of bubbles, and (iii) simplified implementation of the electrolyzer by direct humid air-based operation. Given these advantages, developing efficient water-vapor electrolyzers that can extract water directly from humid air can simplify the operation of current electrolyzers, and enable devices for applications where access to deionized water is challenging.

Moreover, by implementing electrolyzers using microfluidic platforms, hydrogen could be used as an energy storage vehicle for portable electronics, only requiring humid air and electricity to be charged. Lastly, the same platform could be easily extended for wireless solar-hydrogen production by the implementation of photoactive electrodes. See Xiang et al., “Modeling an integrated photoelectrolysis system sustained by water vapor,” Energy & Environmental Science, The Royal Society of Chemistry, 2013, Vol. 6, pp. 3713-3721. See also Singh et al., “Design of Membrane-Encapsulated Wireless Photoelectrochemical Cells for Hydrogen Production,” Journal of The Electrochemical Society 2014, Vol. 161, p. E3283-E3296. See also Reece et al., “Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts,” Science 2011, Vol. 334, pp. 645-648. Also, these platforms can be used as fuel cells by operating them in reverse.

Regarding the background art, existing vapor-phase water electrolysis approaches have made use of macroscopic membrane-electrode assemblies. For example, proton exchange membrane electrolysis using water vapor as a feedstock have been described in U.S. Pat. Pub. No. 2013/0092549, Patent Cooperation Treaty Publication No. WO2012/135862, European Patent Application No. EP2694702 A1, and in the publication from Joshua M. Spurgeon and Nathan S. Lewis, “Proton exchange membrane electrolysis sustained by water vapour,” Energy Environ. Sci., 2011, Vol. 4, pp. 2993-2998.

Other approaches of the background art involve light-driven water-vapor splitting systems, for example the vapor phase hydrogen generator described in U.S. Pat. No. 4,522,695, and in the publications of J. Rongé, S. Deng, S. Pulinthanathu Sree, T. Bosserez, S. W. Verbruggen, N. Kumar Singh, J. Dendooven, M. B. J. Roeffaers, F. Taulelle, M. De Voider, C. Detavernier and J. A. Martens, “Air-based photoelectrochemical cell capturing water molecules from ambient air for hydrogen production”, RSC Adv., 2014, Vol. 4, pp. 29286-29290.

Similar microfluidic systems that use liquid feeds had been demonstrated previously, see Modestino et al., “Integrated microfluidic test-bed for energy conversion devices,” Phys. Chem. Chem. Phys., 2013, Vol. 15, pp. 7050-7054.

However, despite all these advancements in the field of water electrolysis systems, still further improvements and novel solutions are desired.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a device structure and physical mechanism is provided to generate hydrogen from water in the vapor phase, such as ambient air humidity, by providing an electrical input. This vapor-fed electrolysis system allows for water to be extracted from the vapor phase into an ion-conducting solid phase material, where it can be diffused towards two electrodes, denoted as anode and cathode, and can be oxidized in the anode to produce oxygen, and can be reduced in the cathode to produce hydrogen.

According to another aspect of the present invention, a system and method for electrolyzing water is provided. The system first includes a set of micro-sized electrodes placed on top of an insulating substrate. These electrodes are covered by a thin-film of a solid state ion-conducting material. In at least one embodiment, the ion-conducting material is a polymer. On top of the ion-conducting thin film, a set of two independent channels are placed directly above of the electrodes, such that the products generated at each electrode are collected in independent channels.

In accordance with another aspect of the present invention, a method for water electrolysis from water vapor streams is provided. Preferably, the method includes the steps of providing a vapor streams flow through channels that are placed over a microstructured array of electrodes, and performing a water splitting reaction with the microstructured array of electrodes. In addition, preferably the method further includes the step of collecting evolved hydrogen and oxygen by a separate microstructured channel for each electrode of the microstructured array of electrodes.

According to yet another aspect of the present invention, these channels can be fabricated of a solid and gas impermeable material, such as epoxy-based negative photoresist, for example but not limited to SU-8. When water-containing vapor is in contact with the device, the ion-conducting material will absorb water. When a potential higher than 1.23 V is applied between the micro-sized electrodes, the water that is absorbed in the solid-state conductor and in direct contact with the surface of the electrodes will be dissociated into hydrogen and oxygen at the electrodes. In particular, water is dissociated into oxygen and protons at the anode, and protons migrate to the cathode where they are reduced to hydrogen. Therefore, overall water is dissociated into hydrogen and oxygen at the electrodes. The positive ions generated in the anode, will migrate towards the cathode through the ion-conducting film. The hydrogen and oxygen generated in the surface of the electrodes will diffuse through the ion-conducting film and reach the channels placed above each electrode. The substrate on top of which the electrodes are deposited can be, but not limited to silicon dioxide. The electrodes material can be but not limited to platinum, iridium oxide, or ruthenium oxide. The width of the electrodes can be in the range, but not limited 0.1-1000 μm. The separation between electrodes can be, but not limited to 0.1-1000 μm. The solid-state ion-conducting film can be but not limited to Nafion®. The thickness of the solid-state ion-conducting film can be but not limited to 10 nm-10 μm. The material used to fabricate the channels can be, but not limited to SU-8 or Silicon dioxide.

Further, according to still another aspect of the present invention, a vapor-phase water electrolysis system is provided that can be powered by a photovoltaic device or a wind turbine. In at least one embodiment, it is possible to incorporate photovoltaic materials as one of the electrodes, so as to only require sunlight to split water from vapor-phase feeds.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description with reference to the attached drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention.

FIG. 1 is a cross-sectional schematic perspective representation of a water-vapor splitting device showing parallel electrodes where potential is applied, with a solid-state ion-conductor covering them, according to one aspect of the present invention;

FIG. 2 is a top view representation of the channel assembly for the water-vapor splitting device, with the channels arranged in a double spiral architecture, according to another aspect of the present invention;

FIG. 3 is a flow diagram for the fabrication process for manufacturing a water-vapor splitting device, showing different stages of a method, according to another aspect of the present invention;

FIG. 4 is a detailed computer aided design (CAD) diagram for the microstructured electrodes used for the water-vapor splitting device, according to still another aspect of the present invention;

FIG. 5 shows graphs with a set of current-voltage characteristics of the water-vapor splitting device for different thickness of the ion-conductor film, according to yet another aspect of the present invention;

FIG. 6 shows graphs with a set of current output curves of the water-vapor splitting device containing a 733 nm Nafion® ion-conductor film operated at different feed flow rates, according to another aspect of the present invention; and

FIG. 7 shows graphs depicting a gas chromatography trace for hydrogen at hydrogen evolution channels and oxygen evolution channels of the water splitting device, according to still another aspect of the present invention.

Herein, identical reference numerals are used, where possible, to designate identical elements that are common to the figures. Also, the images in FIGS. 1 and 3 are simplified for illustration purposes and may not be depicted to scale.

DETAILED DESCRIPTION OF THE SEVERAL EMBODIMENTS

FIG. 1 is a cross-sectional schematic perspective view of the water-vapor splitting device or water-vapor fed microfluidic electrolyzer device according to one aspect of the present invention, showing parallel electrodes 101 across which a voltage potential can be applied, with a solid-state ion-conductor 100 that covers the parallel electrodes 101, and a channel structure 102 that is configured for collecting the gases that are produced by the water-splitting process. Preferably, the electrodes are micro-sized. Exemplary, the solid-state ion-conductor 100 can be made of a Nafion® thin film, and the parallel electrodes 101 can be made of Platinum. A set of channels 102 are arranged to collect the products emanating from each of the electrodes 101. As shown in FIG. 1 where a cross-sectional view of two parallel channels 102 of the device are given, the flux of chemical species and charges is shown schematically with the arrows. The channel structure 102 can be realized as SU-8 channels. Species that are indicated by reference numeral 103 can correspond to hydrogen gas, and species that are indicated by reference numeral 104 can correspond to water vapor, and the species that are indicated by reference numeral 105 can correspond to oxygen gas. Reference numeral 106 corresponds to electrons going into and out of the parallel electrodes 101, and reference numeral 107 denotes protons migrating through the solid-state ion-conductor 100, for example a Nafion® thin film.

The water-vapor fed microfluidic electrolyzer device of FIG. 1 can perform a water splitting process, according to another aspect of the present invention. When a voltage potential is applied to the electrodes 101 that is sufficiently high to overcome the thermodynamic potential for water splitting, i.e. on or above 1.23 V, charges will flow through the electrodes 101 to perform the electrolysis process. In other words, when a voltage potential that is higher than 1.23V is applied between the anode and the cathode of the two parallel electrodes 101, electrical current starts to flow between the electrodes 101. Electrons are extracted from water molecules in the anode, as water is oxide into oxygen and protons. Protons then flow through the solid-state ion conductor film 100 towards the cathode where they combine with electrons to generate hydrogen molecules. The water required for the reaction that takes place in the anode diffuses through the solid-state ion conductor film 100 from the vapor phase. The water vapor can be fed into the microfluidic electrolyzer device by, but not limited to, forced convection using a micropump or by natural convection from the environment. The generated gases, oxygen in the anode and hydrogen in the cathode, diffuse from the electrodes 101, through the ion conductor, and into the vapor stream above it.

Vapor streams flow through channel structure 102 that are placed aligned over the electrodes 101. According to at least one embodiment of the present the invention, the channels can be fabricated with SU-8 by the use of photolithography. A vapor stream flows through each channel 102 independently over the length of the electrodes 101 until they are collected in an outlet of the device (not shown).

FIG. 2 is a top view of an exemplary channel assembly that includes the channel structure 102. In this view, the channels have been made visible by means of a dark dye solution 201 and a light dye solution 202 that respectively flow independently through the set of parallel channels 102. The set of parallel channels 102 that form the channel structure is arranged in a double spiral architecture. The reference numeral 101 is not shown in FIG. 2 for better readability. In this figure, the set of channels with colored dyes are circulating independently, in a similar manner as gas streams would be during normal operation of the device. In at least one embodiment of the device, the hydrogen generated by the water-vapor fed microfluidic electrolyzer device can be used directly into a fuel-cell or stored in a tank for use at a later stage.

FIG. 3 shows a schematic representation of a flow diagram for the fabrication process of the water-vapor splitting device, showing different stages of a method, according to another aspect of the present invention. A process is shown that is used for the fabrication of the electrodes, the channel structure, and the steps for the assembly of the water-vapor splitting device. The electrode manufacturing process may begin by providing a silicon substrate 301 that includes a layer of silicon oxide, located within the silicon substrate, and creating a positive photoresist 302 onto the silicon substrate 301 by using a photolithography step 310. A thin-film platinum layer 303, with either Titanium or Chromium as seed layers, is then deposited over the positive photoresist 302 and the silicon substrate 301, by using a physical vapor deposition process 311, either by evaporation or sputtering, followed by a lift-off process 312, to obtain the electrodes 101. For example, a silicon wafer with a silicon oxide layer thermally grown is lithographically patterned with a combination of LOR and AZ1512 photoresists, and using a Chrome mask with the layout pattern of a double spiral structure presented in FIG. 4. Then, a thin film of Titanium and Platinum is deposited over the patterned substrate 301. This film can be, but not limited to, 20 nm in thickness for Titanium and 200 nm in thickness for Platinum, and can be deposited, but not limited to, via electron beam evaporation. The photoresist is then lift-off with step 312 using a developer solvent and the metal pattern as the electrodes 101 remains attached to the substrate.

The channel structure manufacturing process can start by providing a glass wafer 304 as substrate for the channel structure 102, and realizing thereon by means of photolithography 313 the material 305 that is used for the channel fabrication, in the variant shown SU-8. At this stage of the method of manufacturing the water-vapor splitting device, a Nafion® thin film 306 is deposited on top of the substrate 301 and the thin film of platinum 303 that will be forming the electrodes, by means of a spin casting process 314 for Nafion®. It is understood that the use of Nafion® is only a preferred embodiment for the solid-state ion conductor. In a variant, a thin-film 306 of solid-state ion conductor can be casted over the substrate 301 using a spin-coater. In at least one embodiment of the present invention, when the solid-state ion conductor is made of Nafion®, it can be casted from a polymer dispersion in a mixture of water and aliphatic alcohols. Next, an alignment and bonding process 315 between the electrode carrying substrate and the channel carrying structure is then performed in another assembly step.

Furthermore, in at least one embodiment of the device, the channels 102 are photolytographically patterned in a separate glass wafer using SU-8 as the photoresist. Inlet and outlet ports are placed in the device prior to bonding with the wafer containing the patterned electrodes 101. In at least one embodiment of the invention, the inlet and outlet ports are drilled and have a diameter no greater to 2 mm. Then, the channels 102 patterned in a glass wafer are aligned with the electrodes 101 deposited in the silicon oxide-silicon wafer such that each channel 102 lies directly on top of the electrodes 101. The aligned wafers are brought up to 200° C. and bonding between the SU-8 channels and the solid-state ion conducting film is achieved in less than 5 minutes. Subsequently, a Polydimethylsiloxane (PDMS) structure is placed on top of the inlet and outlet ports and serves to facilitate the installation of injection needles for the feeding and collection ports.

FIG. 4 is a detailed computer aided design (CAD) diagram for the microstructured electrodes used in the device fabrication, showing a double spiral architecture of the parallel electrodes.

The principle of operation has been proven by a series of experimental results and measurements. As a proof of principle of operation, the devices fabricated based on the methods described above were operated electrically, by contacting independently each of the electrodes with a potentiostat. FIG. 5 is a set of current-voltage characteristics of the device for different thickness of the ion-conductor film, showing current increasing as a function of applied potential when the device is equilibrated under an air environment with 100% relative humidity. Cyclic voltammetry traces of patterned electrodes are shown, equilibrated with 100% relative humidity air and containing films of Nafion® of different thicknesses on top of them. The results demonstrate that initial current levels above 3 mA are achievable with applied potentials below 3V.

FIG. 6 is a set of current output of the device containing a 733 nm Nafion® ion-conductor film operated at different feed flow rates. The feed used in these measurements was air with 100% relative humidity. Complete devices were operated at constant applied potential of 3 V with a Nafion® film thickness of 733 nm under different flow rates. The performance of the devices suffers an initial drop in current density and then equilibrates at a steady current level, which for the case of fast flow rates (15 mL/hr) approaches 1 mA of current.

FIG. 7 is a gas chromatography trace showing high hydrogen concentration in the effluent from the hydrogen evolution channels and low hydrogen concentration from the oxygen evolution channels. To demonstrate that the current measured in the devices was being used for the production of hydrogen, gas chromatography measurements were performed in the effluent from channels covering the cathode and anode respectively. The gas chromatography results show that more than 96% of the hydrogen produced by the device is collected in the channels placed directly above the cathode side.

While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments, and be given the broadest reasonable interpretation in accordance with the language of the appended claims. 

1. A system for water electrolysis from water vapor streams, the system comprising: a microstructured array of electrodes for water splitting reaction.
 2. The system of claim 1, further comprising: independent microstructured channels for collecting evolved hydrogen and oxygen from the microstructures array of electrodes.
 3. The system of claim 1, further comprising: a thin layer of solid-state conductor located on top of the microstructured array of electrodes to provide pathways for ionic conduction, water absorption from vapor phase, and hydrogen and oxygen diffusion.
 4. The system of claim 1 where the microstructured array of electrodes include photovoltaic components so that a potential necessary for water electrolysis is at least partly provided by the photovoltaic components upon light irradiation.
 5. The system of claim 1, wherein the microstructured array of electrodes is operated by an external electrical bias from an electrical power source.
 6. A method for water electrolysis from water vapor streams, comprising the steps of: providing a vapor streams flow through channels that are placed over a microstructured array of electrodes; and performing a water splitting reaction with the microstructured array of electrodes.
 7. The method according to claim 6, further comprising the step of: collecting evolved hydrogen and oxygen by a separate microstructured channel for each electrode of the microstructured array of electrodes. 